Obtaining citizenship from father

I am a British national born in the UK to an English mother. My father is Saudi and has resisted me obtaining citizenship.
I am also married to a Saudi national.
I want to apply for my Saudi nationality (whilst keeping my British nationality). My father married my mother in the late 1970s/early 1980s in the UK but there is no certificate of marriage. I have visited the Kingdom 4 times and am recognised by the family as his son.